The invention relates to a method of manufacturing an aperture plate or diaphragm, more particularly, a method suitable for manufacturing an aperture plate with a high precision to be used for an electron beam apparatus, such as an electron microscope.
Heretofore, in an electron beam device, such as an electron microscope, an aperture plate for controlling a diameter of an electron beam has been used. The aperture plate is formed such that a micro-hole is provided on a plate made of high melting point metal, such as molybdenum, wherein a coating of platinum or platinum palladium is applied on the surface of the aperture plate to prevent electrical charge and contamination, for example, as disclosed in Japanese Patent Publication (TOKUKAIHEI) No. 04-206244.
As described above, in the conventional aperture plate, an etching has been used as a method for making the micro-hole on the aperture plate. A resist for the etching of the molybdenum plate must stand high heat, corrosion and acid. Thus, the resist is different from a resist used for production of the normal semiconductor. However, there have been problems such that if the resist is not completely removed to thereby remain on the surface of the aperture plate, the resist residue, which is an insulating material, is electrically charged on its surface. When the aperture plate is mounted on an electron microscope, the charged aperture plate causes a negative effect on the electron beam, and also the resist residue becomes a contamination, i.e. impurity source, which lowers the microscope resolution.
Also, there has been a thermal damage problem. Even if platinum is coated on the aperture plate, the resist residue is easy to vaporize by the heat created by the electron beam, and the coating peels off. As the resist is an insulating material, if the resist residue vaporizes and condenses on the surface, the surface holds electron charge, and interferes the electron beam.
To solve the problems, the aperture plate is cleaned by a novel cleaning method and is further coated by osmium. These techniques are disclosed in Japanese Patent Publications No. 11-030364 and No. 11-208534. In the techniques of the publications, it has been confirmed that application of an osmium coating to the aperture plate is extremely effective for eliminating the electron charge and contamination of the aperture plate due to high second ionization energy of osmium.
Plasma spattering and plasma excitation chemical vapor deposition (CVD) are used as a method for coating osmium on the aperture plate. For example, in plasma spattering, rare gas plasma is generated; charged particles from the rare gas collide against osmium; and osmium atoms are accumulated on the aperture plate.
However, according to the plasma spattering method, it is difficult to form a uniform osmium layer inside a micro-hole since the osmium atoms are accumulated according to its physical mechanism. Since a coating layer structure is determined by a topological angle of an opening portion of the micro-hole, it is difficult to form a uniform layer to cover an interior of the micro-hole.
On the other hand, according to the plasma excitation CVD, a precursor is transferred by a gas containing osmium and reacted on a surface of a plate. Thus, the precursor can reach the interior of the micro-hole, so osmium can accumulate the interior of the micro-hole.
In the plasma excitation CVD, an osmium oxide sublimation gas is used from the standpoints of a vapor pressure and an impurity contained therein. However, since the sublimation gas contains oxygen, oxygen contaminates the coated layer as an impurity.
Using the techniques in the prior publications, an electrically conductive amorphous osmium having extremely small oxygen content can be obtained by controlling a structure, base plate temperature, gas pressure and power of the plasma generating device. Also, a dense osmium coating with extremely low oxygen content can be obtained by annealing.
However, since a chemical bonding between oxygen and osmium is strong, a trace amount of oxygen still remains in the coating. Therefore, the anti-static character of the coating is deteriorated, and it is still very difficult to achieve consistent characteristics of the respective apertures, which greatly affects the repeatability of the product.
An object of the present invention is to solve the problems in the conventional techniques as described above and to provide a method of manufacturing an aperture plate, wherein a good repeatability can be obtained.
Another object of the invention is to provide a method of manufacturing an aperture plate, wherein an electrically conductive amorphous coating having a high density and purity can be formed to an interior of a micro-hole, and an aperture plate with such high quality coating can be produced.
Further objects and advantages of the invention will be apparent from the following description of the invention.